subscripts

Subscripts are characters that are set below the baseline and smaller than the surrounding text. They are used to convey additional information without expanding the line height, such as indices, chemical formulas, or mathematical notation.

In mathematics, subscripts indicate indices of elements in sequences or components of vectors and matrices. Examples include a_i as the i-th element of a sequence a, or x_j as the j-th component of a vector x.

In chemistry, subscripts specify the number of atoms of an element in a molecule, as in H2O or CO2.

In computing and data notation, subscripts denote array indices or components of data structures, e.g., A[i] or x_k. They appear in algorithms, tensors, and scientific notation.

Rendering and encoding: Subscripts are produced by typographic features in word processors and via the HTML <sub> element. Unicode provides a limited set of subscript characters (for digits and a few letters); for full alphabets, use formatting or images. Subscripts can affect line height and alignment, and accessibility considerations apply for screen readers.

Subscripts should be distinguished from superscripts, which are placed above the baseline and are used for exponents and footnotes.